There is ancient folklore from all over the place that talks about witches possessing the power the shapeshift or transform themselves into any form they want, usually some kind of animal (the kinds of animals differs according to where the lore originated). Most of these stories claim the witch gains the power through her familiar, which also strengthens the witch’s spell craft work.
Because the stories all originate from different locations around the world I don’t think it’s far fetched to assume this is (or at least was at one time) a possibility. Of course one would have to assume it’s most likely the witch’s spirit which leaves her body and either enters a vessel (the body of an animal) or perhaps physically manifests itself rather than the witch herself transforming like in a warewolf movie. The laws that govern physical reality still apply to witches.